The present invention relates to an image density control apparatus for copying machines, and more particularly to an image density control apparatus for a copying machine having a mechanism for projecting a predetermined density pattern on a photoconductive member through an image projecting optical system, developing the resulting pattern image, detecting the density of the developed image on the surface of the photoconductive member by density sensor means and controlling the density of the image to be formed on the photoconductive surface in accordance with the detected density, and a magnification control system for varying the magnification of the image to be projected on the photoconductive surface by controlling the image projecting optical system.
Such an image density control apparatus controls automatically the density of the image constant always and is already known as disclosed, for example, in U.S. Pat. No. 4,416,535.
This U.S. Pat. No. 4,416,535 solves the disadvantage which arises in case that a self-bias system and above mentioned image density control system are parallelly employed for a magnification changeable copying machine. Namely, the size of the density pattern projected on the photoconductive member through the image projecting optical system varies due to the magnification, and an effective developing bias voltage controlled by the self-bias system also varies causing in the amount of the toner adhering to the area of latent image of the pattern to vary also, so that the toner replenishing can not be correctly controlled. The apparatus of U.S. Pat. No. 4,416,535 is characterized in that the reference toner amount in the toner replenishing control system and/or voltage of a developing bias power source is changed according to the magnification. However this apparatus tends to become complicated in structure.
On the other hand, even if the problem based on the self-bias system is set aside, another problem still remains. Namely, the area of the pattern formed on the photoconductive surface alters with the magnification since the pattern is projected on the photoconductive member by the image optical system.
Because the pattern on the photoconductive member is developed and then checked for density, the amount of toner used for development varies with the variation in the area of the projected pattern. Further because the pattern image formed on the photoconductive member must have a certain area for accurately detecting the density, there is the need to assure the required area even at the smallest copy magnification. Accordingly when making life-size copies or enlarged copies, the pattern image becomes excessively larger than is needed to entail the objection that an excess of toner is used for developing the pattern image. Such objection becomes more pronounced when the copying machine is adapted for density control in operative relation to the copying operation, for example, for every copying cycle or for every copy of odd ordinal number during continual operation.